Objects created with a specific size aren't coming out at that size when exported
I had the exact same problem.  



First, make sure your scale on the object is 1:1.

I Highly (x10!) recommend you abandon all thoughts of using English/Imperial units and go to millimeters.

In Properties Panel, Scene, Units, set Metric Unit to 1.000.  Length=Milimeters 

The second image is the key that will open the city.   That little "Scene Unit" checkbox (Always check to make sure this is ON!).   

Me?   I save these settings as a default startup file so I have these settings are correct to begin with.

Oh!   On the second figure... don't forget to also inspect the "Include, Selection Only checkbox"!

I use the Measure-It tool (to aid in displaying key dimensions 'say' overall height and width for example.

I personally export to Stl files.    On the 3D printer side of things, I recommend Simplify 3D .   It's good investment.  I forget the $ but it isn't much.    Stay clear of nylon filament (strong but doesn't stick to the build plate) but choose Raptor and other "Print in place" type filaments.   Raptor is stronger than ABS.  PLA is weak but good for practice runs.